PHILADELPHIA (CBS) – With summer over, the kids are heading back to school, and health experts are warning parents about packing safe lunches.

Many lunches are not being kept at the right temperature. One study found unsafe temperatures in 90 percent of home packed lunches among preschoolers tested. That could cause a nasty case of food poisoning–something you want to avoid.

Most brown bags have now been replaced with insulated carriers to maintain food temperatures, which is especially important for perishable items like cold cuts and yogurt. These need to be kept below 41 degrees. Anything above that, bacteria can grow and potentially cause food poisoning.

In a non-scientific test, four identical lunches were packaged in different containers, two types of insulated bags, an un-insolated lunch box and a plain old plastic bag.

After being chilled overnight, they were left out in a room at 74 degrees for four hours.

At 72 degrees, one yogurt was well above the safe level, as were most of the items tested.

“The coldest we measured was close to 60 degrees. That, at that temperature you can, you can envision there will be plenty of bacteria growth really rapidly,” said Dr. Francisco Diez-Gonzales, a food safety expert.

The warmer the perishable items, the higher the danger for causing an upset stomach.

One alternative is to pack lunch with items that are not perishable–like peanut butter, pretzels, fruit–or use ice packs to make sure cold items stay cold.

Another trick nutritionists suggest is freezing a water bottle over night, then packing it with lunch. It will keep the items cold and gradually melt, so the student can drink it with lunch or later.